1. Field of the Invention
The present invention relates to a gas sensor, a method for controlling the same, a gas concentration controller, and a method for controlling gas concentration, used to measure oxides such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as CO and CnHm.
2. Description of the Related Art
In recent years, exhaust gas, which is discharged from vehicles or automobiles such as gasoline-fueled automobiles and diesel powered automobiles, contains nitrogen oxides (NOx) such as nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2), as well as carbon monoxide (CO), carbon dioxide (CO.sub.2), water (H.sub.2 O), hydrocarbon (HC), hydrogen (H.sub.2), oxygen (O.sub.2) and so on. In such exhaust gas, about 80% of the entire NOx is occupied by NO, and about 95% of the entire NOx is occupied by NO and NO.sub.2.
The three way catalyst, which is used to clean HC, CO, and NOx contained in the exhaust gas, exhibits its maximum cleaning efficiency in the vicinity of the theoretical air fuel ratio (A/F=14.6). If A/F is controlled to be not less than 16, the amount of produced NOx is decreased. However, the cleaning efficiency of the catalyst is lowered, and consequently the amount of discharged NOx is apt to increase.
Recently, in order to effectively utilize fossil fuel and avoid global warming, the market demand increases, for example, in that the discharge amount of CO.sub.2 should be suppressed. In order to respond to such a demand, it becomes more necessary to improve the fuel efficiency. In response to such a demand, for example, the lean burn engine and the catalyst for cleaning NOx are being researched. Especially, the need for a NOx sensor increases.
A conventional NOx analyzer has been hitherto known as an instrument for detecting NOx. The conventional NOx analyzer is operated to measure a characteristic inherent in NOx, based on the use of chemical luminous analysis. However, the conventional NOx analyzer is inconvenient in that the instrument itself is extremely large and expensive.
The conventional NOx analyzer requires frequent maintenance because optical parts are used to detect NOx. Further, when the conventional NOx analyzer is used, any sampling operation should be performed for measurement of NOx, wherein it is impossible to directly insert a detecting element itself into a fluid. Therefore, the conventional NOx analyzer is not suitable for analyzing transient phenomena such as those occur in the exhaust gas discharged from an automobile, in which the condition frequently varies.
In order to dissolve the inconveniences as described above, there has been suggested a sensor for measuring a desired gas component in exhaust gas by using a substrate composed of an oxygen ion-conductive solid electrolyte.
The suggested conventional gas sensor is exemplified by an all-range type oxygen sensor as shown in FIG. 12. Further, a gas sensor for measuring NOx is also known, with which a gas (for example, NOx) including bound oxygen is measured by lowering the oxygen concentration in the gas to a constant low level by using an oxygen pump, and then further lowering the oxygen concentration to decompose NOx so that oxygen produced during the decomposition is measured by using an oxygen pump.
For example, the gas sensor shown in FIG. 12 will be explained. In this gas sensor, a direct current voltage to be applied to an oxygen pump 104 is subjected to feedback control so that a voltage of electromotive force generated between a measuring electrode 100 and a reference electrode 102 is maintained to be constant. In general, the feedback control is performed by comparing a comparative voltage as a target with the electromotive force generated between the measuring electrode 100 and the reference electrode 102 by using a comparator, amplifying a difference produced by the comparator to generate an amplified voltage corresponding to the difference from the target value, and applying the amplified voltage to the pump.
However, the conventional gas sensor involves problems concerning the following two point. Firstly, for example, when the gain of the amplifier is set to be excessively large in the gas sensor shown in FIG. 12, the feedback control suffers oscillation (first problem). Secondly, when the measurement gas has a high oxygen concentration, it is impossible to accurately measure the oxygen concentration (second problem).
At first, the first problem will be specifically explained. The first problem is caused by the existence of any geometrical dimension of the measuring electrode 100 and a pumping electrode 108 contacting with an internal space 106. For example, when the oxygen concentration around the measuring electrode 100 is lower than the target value, the feedback control is performed so that the pumping voltage Vp is increased. Accordingly, the pumping voltage Vp is increased, the oxygen in the internal space 106 is pumped out, and the oxygen concentration in the internal space 106 is gradually decreased. However, the decrease in oxygen concentration is transmitted to the measuring electrode 100 in a delayed manner due to the presence of the geometrical dimension described above. As a result, the oxygen concentration in the internal space 106 becomes lower than the target value. The lower oxygen concentration is detected by the measuring electrode 100 after a short delay period, and then the feedback control is performed so that the pumping voltage Vp is decreased.
In this case, the partial pressure of oxygen in the internal space 106 is gradually increased in the same manner as described above. However, a phenomenon occurs due to the geometrical dimension, in which the oxygen concentration in the internal space 106 has been excessively increased when the measuring electrode 100 detects the increase. As a result, the feedback control circuit suffers oscillation.
In order to solve this problem, if the gain of the amplifier is decreased, a state of insufficient control occurs when the oxygen concentration in the measurement gas is increased, because of the following reason. Namely, when the oxygen concentration in the measurement gas is increased, it is necessary to use a large pumping voltage Vp. However, the pumping voltage Vp cannot be increased to a desired valued because of the small gain.
Next, the second problem will be explained. In general, the limiting current type oxygen sensor based on the use of the oxygen pump is exemplified by widely known sensors as shown in FIGS. 13 and 14, in which a constant pumping voltage Vp is applied between an air electrode 110 and an electrode 112 disposed on the side of exhaust gas so that the oxygen concentration is measured based on a value of a current flowing therebetween. Upon the operation of such a sensor, the constant pumping voltage Vp is applied. Therefore, for example, when the oxygen concentration is increased, the amount corresponding to electromotive force is decreased by the amount corresponding to impedance of the oxygen pump. As a result, an oxygen concentration to be substantially controlled is increased. In such a situation, it is impossible to accurately measure the oxygen concentration (the oxygen concentration is higher at Point B than at Point A in a characteristic curve shown in FIG. 15).
On the other hand, Japanese Utility Model Publication No. 7-45004 discloses a system in which a voltage corresponding to a pumping current is generated by using an operational amplifier. The voltage is returned to the operational amplifier via a feedback resistor, and it is supplied to a resistor which is connected to a power source in series. When the pumping current is increased, the voltage generated by the resistor is superimposed and applied to the pump.
This system comprises a circuit as shown in FIG. 16. The output of the operational amplifier OP is returned to an input terminal on a side of an air electrode 110 via the feedback resistor R1 so that the voltage corresponding to the pumping current is generated at an output point A. On the other hand, the output is returned to an input terminal of an electrode 112 disposed on the side of exhaust gas via the resistor R2, and the current is allowed to flow via the resistor r so that an amount of voltage generated in the resistor r is superimposed on a power source voltage V.sub.E.
When the resistor connected to the power source in series is appropriately designed, a voltage corresponding (actual pump impedance x pumping current) is superimposed on the pumping voltage Vp so that the operation point is set at any of certain flat portions on limiting current characteristic curves as shown in FIG. 17. Thus the oxygen concentration is measured with a high degree of accuracy.
However, when the oxygen concentration in a measurement gas is increased, the amount corresponding to voltage drop is increased, and it becomes far larger than the amount corresponding to electromotive force. Therefore, it is difficult to operate the gas sensor at an operation point which accurately corresponds to a certain electromotive force.
When the temperature of exhaust gas greatly changes as in the automobile, the sensor is provided with a heater, for which a mechanism for controlling the electric power to be supplied to the heater is provided, in some cases. Even when such a system is adopted, the impedance of the oxygen pump is slightly changed. When the pumping current is increased, a large error occurs in correction for the amount corresponding to voltage drop. As a result, it is difficult to correctly measure the high oxygen concentration.
This problem is most serious especially when the pump is used as an oxygen concentration controller. When the pump is used as an oxygen sensor, even if the oxygen concentration in the measurement gas is increased, the pumping current is increased, and the oxygen concentration in the measurement space is increased from 10.sup.-10 atm to 10.sup.-3 atm, then the change in current based on the change in oxygen concentration is about several % at most, as compared with the increased pumping current. However, when the pump is used as an oxygen concentration controller, the change in oxygen concentration is exactly the large change from 10.sup.-10 atm to 10.sup.-3 atm as it is.
As described above, the conventional gas sensor involves the first problem that the feedback control system suffers oscillation when the voltage applied to the oxygen pump is controlled on the basis of the electromotive force between the measuring electrode and the reference electrode, and the second problem that it is impossible to accurately absorb the error of the amount corresponding to the voltage drop resulting from the impedance of the oxygen pump.